electrical stumper question

[rob'smobile]

I highly doubt anyone will have an accurate answer ,but here it is .these pics are of the ballast that makes the tach work ,. 1969 911 t 2.0 ltr ,I have posted for a replacement unit but have nothing yet., The ohm readings are in the pics ,.and what I want to know is if any body has info on the correct readings and or if the ones I have are correct ?"if not is it possible for me to build my own unit ? I just want my tach to work . The schematic is very simple . term A is ground term B is signal from coil term C is the signal to the tach simple ,. but I have signal going in but nothing coming out,. thanks Rob,, feel free to pm me

[dhoward]

That's odd. My 69 T had a rectangular box that drove the tach.....

[rob'smobile]

can you post a pic of your box ? I am not even sure this is the correct part for this car ? thanks Rob

[dhoward]

Here ya go!

[rick-l]

Just looking at the picture this looks like an L-C filter to smooth out the ringing from the coil. Doing reverse engineering from the pictures it looks like the signal comes in, goes into the resistor, through the coil, to the output which has a cap to ground.

If this is how it is wired the first component I would suspect is the cap since it should eventually show an open when measured with an ohm meter.

I can't tell what colors the resistor bands are, but that is a cheap component also.

Edit: Ooops hey wait a minute, how many wires come out of that coil? Transformer for level shift maybe?

[rick-l]

Come on... I'm dying of curiosity as to what this does. Since the two resistances add up to the larger pne it must be a transformer.

[rob'smobile]

the band colors from top to bottom are
gold

lt brown

green

lt brown ,maybe tan ,or very light gray ?

I don't have the rectangular box , but I do have an open 6 way plug ,I was told it may be a CDI box ,and was also told I don't need one as the engine does run .I think I am missing the neccasary items to make this correct , because it's a 69 911 t it may or may not need a cdi box ? what do you think ?thank you Rob

[Bob Ashlock]

I know this is a very old thread but happened across it and wanted to add a comment. The main reason the ballast unit was needed is that the older tachometers were looking for a positive high voltage 'spike' like they would see from a traditional Kettering (points, coil, condenser) ignition setup. The tach was triggered by the points/coil signal connection. When the CDI was introduced, the coil signal changed. The CDI discharge produces a high voltage NEGATIVE voltage spike at the coil terminal. Tach won't see it. Enter the ballast ('intermediate unit' as Porsche called it). That simple unit takes the negative coil signal and with a little LCR network 'flips' it into a positive going spike, sends it onward to the tach which recognizes it again. That's it. No more magic than that.